Existing commercial transport jet aircraft typically include two or more primary turbine engines for propulsion. These aircraft also typically include at least one auxiliary power unit (APU) that provides electrical and/or pneumatic power in addition to or in lieu of the power provided by the primary engines. Accordingly, APUs can be used to provide power to the aircraft when the primary engines are not running, for example, when the aircraft is waiting in an airport gate. The APUs can also provide temporary power to start the primary engines during normal operations, and/or temporary emergency power during an engine-out condition or other emergency condition.
FIG. 1 illustrates a system configured in accordance with the prior art, in which an APU 10 provides temporary electrical power to an aircraft electrical system 40. A controller 20 directs the operation of the APU 10, and a control input device 30 (typically housed at the flight deck of the aircraft) allows a pilot or other operator to manually direct the operation of the controller 20 and therefore the APU 10. Accordingly, the control input device 30 can include a rotary knob that is in an “off” position when the APU 10 is unstarted. The pilot or other operator rotates the knob to the “start” position to start the APU 10. Then, the operator releases the knob, which is spring loaded so as to rotate back to the “on” position. The APU 10 remains in a started state with the knob in the “on” position until the pilot manually moves the knob to the “off” position.
In certain aircraft, the controller 20 can automatically shut down the APU 10 in case of significant operating malfunctions. Such malfunctions include a fire in the compartment housing the APU 10, or failure of the APU 10 itself. In certain aircraft, the controller 20 will start the APU 10 automatically only if all electrical power on the aircraft has failed. Accordingly, the APU 10 can provide automatic backup power in this emergency situation. Typically, a manual action is required to start the APU and, in other situations, the APU 10 can be configured to operate during an entire flight. In most situations, the APU 10 need not be operational for the aircraft to be dispatched. If the APU 10 is not operational and additional power is required during flight, the same signal that triggers in-flight emergency starting of the APU 10 can instead trigger deployment of a ram air turbine, which provides additional electrical power during flight.
While the APU system described above provides adequate ground and emergency power for existing aircraft, both airline operators and airline manufacturers have come under pressure to increase the efficiency of overall aircraft operations. Accordingly, it may be desirable to further increase the overall efficiency of aircraft power system and aircraft APUs to reduce aircraft operational costs.